MAC11 update

John Cody Forbes cody at
Thu Sep 11 05:51:43 PDT 2008

There's no such thing as a dumb question, just dumb responces ;-). Using a 
scope at all for the first time is a learning experiance with a steep curve, 
but it sounds like you've already accomplished that one. We suspect that the 
inputs are all ok, but it's worth checking anyways. How many leads it has 
depend on how many channels the scope has. Many (maybe most) scopes have 2 
channels, meaning you can check 2 things at once and it will display 2 
signals on the screen. Each lead will have two wires and at the end two 
clips, one for signal and one for ground as you suspected.

With a 2 channel scope I would start with one channel on the flywheel 
referance sensor (the one towards the rear of the car) and one channel on 
the ignition coil inputs before the darlington transistor (back-probe the 2 
pin AMP connector that is just towards the passenger side of the car from 
the coil). This way you can referance whats going in and whats comming out 
at the same time. If all looks good then switch channel two to the actual 
coil inputs which are two larger wires that plug directly into the bottom of 
the coil (white and blue IIRC). The last part is watching whats comming out 
of the coil and is harder. Most scopes don't come with any sort of inductive 
pickup that you can use on an ignition wire. I think the way you are going 
to have to handle that is either a spare spark plug plugged into the end of 
one of the plug wires so you can view the spark while it's running, or an 
inductive timing light.

While you've got the scope you should go to the fuel side too. Check your 
tach input to the 034 and all of the outputs.


Rick Houck wrote:
> I do have some dumb questions though about using a scope in an
> automotive application. Should I start with the inputs at the ECU
> connector, and the output at the coil? Also, do the scopes pretty
> much have two leads (for ground and signal), or will we use a trigger
> lead? Thanks.
> Rick
> ---- Original Message -----
> From: "Ben Swann" <benswann at>
> To: "'John Cody Forbes'" <cody at>; <quattro at>;
> "'Rick Houck'" <rhouck at>
> Cc: "'Ben Swann'" <benswann at>
> Sent: Wednesday, September 10, 2008 3:34 PM
> Subject: RE: MAC11 update
>> Second that remark - signaling to the coil might be correct, but that
>> amplifier does
>> require a good ground as does the coil.  I invariably put them all
>> into one common point
>> that tie together in a standoff post on firewall with at least #6
>> wire running back to
>> the battery post - even if the car is a giant ground strap itself.
>> You will also want to examine the positive feed to ECU - I noted
>> some of the "stock"
>> wiring to be seemingly marginal to support a load of a coil, yet
>> seems all goes through
>> the ECU - with possible exception of the coil positive feed itself - I 
>> forget how that
>> is, but may be something to check.   However, typical coil feed
>> breakdown would occur at
>> more like 7000 RPM, not at 2K I'd think.
>> Still I believe it to be something else than ignition.  I swore up
>> and down I had
>> ignition problem on my Megasquirted UrQ with MAC-14 ignition and
>> pretty much went
>> through the same things as Rick.  It really turned out to be EFI
>> pulse width was too
>> high - injector duty cycle longer than the engine rev.  In addition
>> I had and still have
>> a serious fuel delivery problem that is not easy to diagnose or
>> resolve. On the fuel delivery issue, I believe - an educated belief with 
>> lots
>> of trial and error
>> - that the tank and pump are not designed to support the type of
>> fuel flow and pressure
>> an EFI system requires.  Basically with the lower pressure of the EFI
>> Rail, the fuel is
>> pulled into the pump faster than the tank can deliver ultimtely
>> resulting in cavitation.
>> Originally the problem reared its head at around 5-6k, but once
>> thing get warmed up, the
>> car will sometimes barely get out of its way - sucking - starving -
>> bucking.  Changing
>> pumps and lines have helped to a degree, but it seems I either need
>> to find a pump that
>> delivers a lower flow, a surge tank setup, or get a completely
>> different tank with
>> submersible pump.  If Rick has not experienced this yet with his
>> setup, I suspect he
>> will as the 4000Q tank/pump are nearly identical to the UrQ.  And it
>> feels like a
>> ignition problem, but not!
>> Ben
>> -----Original Message-----
>> From: John Cody Forbes [mailto:cody at]
>> Sent: Wednesday, September 10, 2008 3:14 PM
>> To: Ben Swann; quattro at; Rick Houck
>> Subject: Re: MAC11 update
>> DING DING DING Ben just found a possibility that I haven't seen
>> suggseted nor checked
>> yet. Check the engine's ground strap for cleanliness and tightness.
>> Then check it again.
>> Maybe, just maybe, the ground is not good enough to allow the
>> ammount of current flow
>> required at higer-than-idle RPM's.
>> -Cody
>> Ben Swann wrote:
>>> Cody,
>>> That is a good clarification - what this also means is that in order
>>> for the primary signal to be generated, both flywheel sensors need
>>> to be present and signaling and they must sync. up with the Hall
>>> sensor in the distributor cap.  This also triggers the fuel pump. From 
>>> then on out, the primary goes through the Darlington
>>> Transistor/signal amplifier mounted on the coil then through the
>>> coil, cap and rotor and out through the wires and plugs to ground. So 
>>> the only things that could be breaking down are coil, cap, wires
>>> or ground. Once again, unlike the earlier cars that used only hall 
>>> signal to
>>> generate primary( five window hall distributor), there is no
>>> ignition timing adjustment - distributor must be in a set position,
>>> which is preferably dead middle of the hall window.  The ECU does
>>> the rest of the timing.
>>> One possibility that breaks this above "rule" down is that the
>>> timing belt has slack and/or window is just on edge so that the
>>> ignition slips out of window.  Usually this reveals itself in a
>>> no-start condition that is temperature related.
>>> So - you might have a breakdown in the spark output side, but I
>>> think you have covered thost bases well.
>>> Ben
>>> -----Original Message-----
>>> From: John Cody Forbes [mailto:cody at]
>>> Sent: Wednesday, September 10, 2008 1:08 PM
>>> To: Ben Swann; 'Rick Houck'; 'Louis-Alain Richard'
>>> Cc: quattro at; 'P Cole'; 'Ado Sigal'
>>> Subject: Re: RE ; MAC11 update
>>> What Ben means to say is that all is well with primary ignition
>>> (signal output to the coil), but the ECU knows nothing about whats
>>> going on in the secondary ignition (everything between the coil and
>>> plugs inclusively) and won't store codes for these areas. Thats
>>> where the scope comes in. Get ahold of that scope, your problem
>>> will be diagnosed for sure.
>>> -Cody
>>> Ben Swann wrote:
>>>> Again - what do the MAC-11 codes say.  If 4 4 4 4 then all is well
>>>> with ignition. 

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